This invention relates generally to scanning systems, and more particularly to such systems wherein a linear detector array is optically scanned in a two-dimensional raster pattern.
Prior art thermal scanning systems such as described in U.S. Pat. No. 3,723,642, issued to Peter Laakman on Mar. 27, 1973, use a short linear array of detector elements oriented parallel to the "line scan" dimension of the raster so that each element optically scans the entire field of view. The detector elements are continuously sampled and their output signals are proportional to their instantaneous thermal energy exposure. The output signals of the various detector element are identical except for noise and a small time delay from each succeeding detector element to the next. By exactly compensating for the time delays and adding the individual output signals a single composite output signal is obtained which has improved sensitivity.
In a known modification of the above system, the detector elements are sampled at discrete intervals and their output signals are proportional to the integral of their instantaneous thermal energy exposure over the interval.
An important aspect of this modification is that the length of the intervals during which the integration occurs is an integral submultiple of the scanning time delay between detector elements so that the corresponding output signals of the various detector elements are identical except for noise and a small time delay from each succeeding detector element to the next. Such thermal scanning equipment has utility in automatic search and acquisition systems where the composite output signal from the array of detectors is electronically processed to detect the presence of a target. In operation, the composite output signal from the array of detectors is passed through a filter which is optimized to match an anticipated target power spectral density. When the output of the filter exceeds a fixed threshold, a threshold crossing sensor indicates the presence of the target. Typical matched filters and threshold crossing sensors are described in chapter 10 and chapter 2, respectively, of Skolnick, Introduction to Radar Systems (1980).
When small-size, distant targets enter the field of view, a problem arises in the use of discrete time sampling sensors, and in particular the modification described above. Since the image of the target may fall on or between the detector elements at the start of a given integration interval, the waveform of the composite output signal produced by the target can vary. This variation means that the automatic processor with its filter and threshold sensor cannot be truly optimized.